E. Giziroglu et al. / Inorganic Chemistry Communications 36 (2013) 199–205
203
and 130 ppm that are typical of aromatic groups. If the resonance struc-
ture (I) were to exist in the solution, proton on the barbituric acid ring
would have been in the two dimensional 1H–13C HETCOR spectrum.
On the other hand, in the regular 13C NMR spectra of H2L2 in DMSO-
d6 solution, two peaks at 88.7 and 115. 2 ppm for H2L2 are attributed
to NH\C\C carbons respectively. Therefore, on the basis of NMR data
we can conclude that the ligands exist in tautomeric form (II) in the so-
lution [40–42]. Another important peaks in the 1H NMR spectrums of
H2L1 and H2L2 in DMSO-d6 solution appear at 11.26 and 10.29 ppm
for the phenolic OH resonance respectively. As expected, OH proton
for H2L1 was shifted to low fields owing to the formation of strong intra-
molecular hydrogen bonding. The 1H NMR spectra of ligands display a
singlet belonging to NH (amide) proton for H2L1 at 11.26 (overlapped
with OH proton) and H2L2 at 11.18 ppm.
Fig. 5. The proposed structure of the [Cu(L2)(DMSO)].
The infrared spectra of H2L1 and H2L2 show four intense carbonyl
bands appearing at 1687, 1642, 1612 and 1582 cm−1 for H2L1 and at
1683, 1628, 1612 and 1590 cm−1 for H2L2. The characteristic amide I
band appears at ~1647 cm−1 for H2L1, but it doesn't appear for H2L2
probably because of overlapping carbonyl stretching frequency. In the
case of H2L1 a very broad peak is observed in the ~3290–2580 cm−1
region which is assigned to the intramolecular H-bonding vibration
(O\H….O) [43,44]. In case of H2L2 the broad medium intensity band
appearing ~3250 cm−1 is assigned to the υ(O\H) vibration. Also the
amide II υ(N\H) stretching band of these compounds are not observed
in the FT-IR spectra probably because of overlapping with broad
medium intensity of OH stretching frequency.
The most significant changes between the IR spectra of the ligands and
their copper complexes were strong υ(S = O) bands at 1090 cm−1
for [Cu(L1)(DMSO)] and 1094 cm−1 for [Cu(L2)(DMSO)], indicating
the presence of the DMSO molecule that coordinates from the oxy-
gen to the metal [45,46].
ascorbic acid (μg AAEs/mg ligand) according to the following equation
obtained from the standard ascorbic acid graph:
Absorbance ¼ 0:0088 ascorbic acid ðμgÞ þ 0:0022:
ðR2 : 0:9991Þ
Ferric reducing antioxidant power (FRAP) assay. The FRAP assay was
determined through a method described by Benzie and Strain [38]
with slight modifications. The FRAP reagent was prepared freshly by
mixing 300 mM acetate buffer (pH 3.6), 10 mM TPTZ and 20 mM ferric
chloride in a ratio of 10:1:1 (v/v/v). Then, 2 mL reagent and 0.1 mL of
sample solution in DMSO (2 mg/mL) were added to test tubes and incu-
bated at 30 °C for 30 min. Absorbance was measured at 593 nm. Trolox
was used as standard and results were reported as equivalents of trolox
(μg TEs/mg ligand) according to the following equation obtained from
the standard trolox graph:
Some of carbonyl absorption band which are appearing in the spec-
tra of metal free ligand, changed to the lower frequency because of the
coordination to the metal. But they could not be assigned since the spec-
tra were complicated with overlaps in the 1500–1700 cm−1 region. On
the other hand, a new band due to C\O vibration appears around
1245 cm−1 for [Cu(L1)(DMSO)] and 1234 cm−1 for [Cu(L2)(DMSO)]
confirm that both complexes are enolate form [47,48].
Absorbance ¼ 0:0816 trolox ðμgÞ–0:0373:
ðR2 : 0:9977Þ
Scavenging activity on 1,1-diphenyl-2-picrylhydrazyl (DPPH). The hy-
drogen atoms or electron donation ability of some compounds were
measured from the bleaching of purple colored methanol solution of
DPPH. The effect of the H2L1 and H2L2 on DPPH radical was estimated
according to Sarikurkcu et al. [39]. 1 mL of ligands solution in DMSO
(0.2-2.0 mg/mL) was added to 1 mL of DPPH radical solution in metha-
nol (final concentration of DPPH was 0.2 mM). After a 30 min incuba-
tion period at room temperature the absorbance was read against a
blank at 517 nm. Inhibition of free-radical DPPH in percent (I%) was
calculated in following way:
Dark green crystals of [Cu(L1)(DMSO)] suitable for an X-ray
diffraction study were obtained by slow evaporation of a saturated
DMSO solution at room temperature about two weeks. The X-ray
structure analysis shows that [Cu(L1)(DMSO)] crystallizes in the
triclinic space group P-1. An ORTEP diagram giving the unique
atom labeling is shown in Fig. 2 and selected bond distance and
angle data are given in Table 1.
The single crystal X-ray diffraction analysis unambiguously
demonstrates the H2L1 that coordinates to the metal center through
ONO donor system. H2L1 acts as dibasic pincer type ligand. The copper(II)
is coordinated in a slightly distorted square planar geometry with the
DMSO which is coordinated over the oxygen atom to the metal center.
All the data for the copper complex are in agreement with those reported
for similar complexes [49,50]. The Cu1\O1, Cu1\O2, Cu1\O3 and
Cu1\N2 bond distances are 1.8984(17) Å, 1.9312(17) Å, 1.8779(17) Å,
I% ¼ 100xðAControl−ASampleÞ=AControl
where, AControl is the absorbance of the control reaction (containing
all reagents except the test compound), and ASample is the absorbance
of the compound tested. BHT and BHA were used as a control.
Results and discussion. Characterization. The barbituric acid
hydrazone ligands were synthesized by refluxing o-hydroxybenzoyl
hydrazine or p-hydroxybenzoyl hydrazine with 1,3-dimethyl-5-
acetylbarbituric acid in the presence of absolute ethanol along
with catalytic amount of glacial acetic acid. The reaction proceeded
smoothly and produced the corresponding ligands in good yields.
All these ligands are air stable, nonhydroscopic and characterized
by MS, FT-IR, 1H, and 13C spectroscopy. Ligands prepared in this
study may exist in three tautomeric forms as shown in Fig. 1.
When we started structural analysis, we have checked two dimen-
sional 1H–13C HETCOR spectrum to decide which tautomeric form
exist in the solution. According to two dimensional 1H–13C HETCOR
in DMSO-d6 solution spectra of H2L2 display two peaks with proton
shifts in the signal range ca 2.7 and 3.2 ppm that correlate with carbon
shifts in the range ca 17 and 28 ppm that are typical of aliphatic –CH3
groups. Another group of peaks with proton shifts in the signal range
ca 6.9 and 7.8 ppm that correlate with carbon shifts around ca 115
Table 5
Antioxidant activity of the ligands by phosphomolybdate and FRAP assaysa.
Sample
Phosphomolybdate assay
FRAP assay
(μg AAEs/mg ligand)b
(μg TEs/mg ligand)c
H2L1
H2L2
270.65
401.99
0.01
13.61
41.43
170.35
1.17
0.39
a
Values expressed are means S.D. of three parallel measurements.
AAEs, ascorbic acid equivalents.
TEs, trolox equivalents.
b
c